Method for coating motor vehicle bodies or parts thereof

ABSTRACT

Method of lacquering a substrate by applying a clear-lacquer coating compound to a precoated substrate or a one-coat top lacquer coating compound to an optionally precoated substrate, followed by curing, the substrate being a motor-vehicle body or parts thereof, in which the substrate is subjected, after the application of the clear-lacquer coating compound or one-coat top lacquer coating compound, optionally to a drying phase and the curing is then performed by irradiation with NIR radiation having the wavelength from 760 to 1500 nm.

BACKGROUND OF THE INVENTION

The invention relates to a method of lacquering, in particular ofmulti-coat lacquering motor-vehicle bodies or parts thereof using, inparticular, water-based lacquers, in particular in the field ofmotor-vehicle repair lacquering.

For ecological reasons, attempts are being made to replace solvent-basedlacquers by water-based lacquers to an increasing extent, even in thefield of motor-vehicle repair lacquering. The water-based lacquersdeveloped have already reached a quality level that makes them equal tothe solvent-based lacquers in a plurality of properties. Some propertieshave, however, not yet reached the quality level of solvent-basedlacquers. For example, the use of water-thinnable undercoat, clear andone-coat top lacquers still presents problems in ensuring.uniformlacquering quality, in particular with regard to surface properties andinterlayer adhesion under varying external conditions. It isparticularly difficult to achieve a reproducible drying of water-basedlacquers and a uniform surface lacquering quality resulting therefromunder ambient conditions involving widely varying atmospheric humidity.

The object of the invention was therefore to provide a method ofmulti-coat lacquering, in particular for motor-vehicle repairlacquering, which makes it possible to achieve coatings having a highand uniform lacquering quality, in particular with regard to surfaceproperties and interlayer adhesion, when water-based undercoat, clearand one-coat top lacquers are used. The uniform quality of thelacquering should be ensured, in particular even under widely varyingambient conditions during application, such as, for example, atmospherichumidity. The lacquering quality should likewise be uniform at criticalpoints, such as corrugations or edges. Furthermore, hard andscratch-resistant coatings should be obtained by the method according tothe invention. Associated therewith is the requirement for a goodcoating polishing capability and, in particular, for a good polishingcapability relatively quickly after application and drying.

It has been found that this object can be achieved by the use, to whichthe invention relates, of NIR radiation for curing clear-lacquer coatsor one-coat top lacquers on motor-vehicle bodies or parts thereof, inparticular in the case of repair lacquering.

The NIR (near-infrared) radiation used according to the invention isshort-wave infrared radiation having the wavelength range from about 760to about 1500 nm, preferably 760 to 1200 nm.

SUMMARY OF THE INVENTION

The invention furthermore provides a method of lacquering a substrate byapplying a clear-lacquer coating compound to a precoated substrate or aone-coat top lacquer coating compound to an optionally precoatedsubstrate, followed by curing, the substrate being a motor-vehicle bodyor parts thereof, which is characterized in that the substrate issubjected, after the application of the clear-lacquer coating compoundor one-coat top lacquer coating compound, optionally to a drying phaseand the curing is then performed by irradiation with NIR radiationhaving the wavelength range from 760 to 1500 nm.

DETAILED DESCRIPTION OF THE INVENTION

The clear-lacquer coating compound or one-coat top lacquer coatingcompound used according to the invention may preferably be water-based.It is, however, also possible to use solvent-based coating compounds.

The method according to the invention may be, in particular, a method ofmulti-coat lacquering. In this case, a top lacquer coating is applied,for example, to the substrate, optionally coated with filler compoundsand/or further coating compounds. Said top lacquer coating may becomposed, for example, of a colouring and/or special effect producingsolvent-based or water-based undercoat coating compound and awater-based clear-lacquer coating compound. The top lacquer coating mayalso be composed of a water-based pigmented one-coat top lacquer-coatingcompound. For example, the procedure may be such that, in the case ofapplication of the top-lacquer coating compound in the form of apigmented one-coat top lacquer, the top-lacquer coat is first subjectedoptionally to a drying phase and then cured by irradiation with NIRradiation and that, in the case of application of the top-lacquercoating compound in the form of an lacquer undercoat/clear-lacquerstructure, a lacquer undercoat is first applied, after curing of thelacquer undercoat or in the wet-on-wet form, optionally after a dryingphase, the clear-lacquer coating is applied, optionally subjected to adrying phase and then cured by irradiation with NIR radiation.

The use of NIR radiation generally to dry paints and lacquers is known.As possible applications, for example, the following fields arementioned: the printing sector, film drying, pipe drying, wood coatings,and powder coatings. As particular advantages of the NIR technology,mention is made of the very rapid drying, in particular in the case ofwater-based lacquers, and the gentle drying as a result of low heatingof the substrate. Nothing is known of the possible applications of thistechnology in motor vehicle lacquering, in particular motor-vehiclerepair lacquering.

Surprisingly, it has now been found that the object of the presentinvention can be achieved by using NIR radiation to cure clear-lacquerand one-coat top lacquer coatings in a multi-coat structure.

The irradiation performed in the method according to the invention withNIR radiation can be performed with a conventional, high-energy NIRsource. Such NIR sources are available commercially (for example fromthe Industrie SerVis company). These are, for example, high-powerhalogen sources having a radiation density of generally more than 1W/cm², preferably more than 10 W/cm², up to, for example, 15 MW/m². Thesources reach, for example, a source surface temperature (incandescentfilament temperature) of over 2500 K, for example of 2500 to 3000 K.Suitable sources have, for example, an emission spectrum with a maximumbetween 750 and 1200 nm.

Preferably, a drying phase is included, according to the invention,before the irradiation with NIR radiation. The drying phase is performedconventionally, for example in air or by blasting with air, for exampleat temperatures of 10 to 80° C., in particular at room temperature. Theair blasted may optionally also be heated. Various blasting systems maybe used, for example hand-held blasting guns, rack-mounted orwall-mounted blasting systems. In the simplest case, drying can beachieved by standing at room temperature.

As a result of including a drying phase, it is possible to avoid blisterformation on the lacquer surface that sometimes occurs with NIRirradiation in the case of direct irradiation after application.

Clear-lacquer coating compounds and one-coat top lacquer coatingcompounds that can be used in the method according to the invention are,for example, conventional water-based clear lacquers and one-coat toplacquers known to the person skilled in the art, such as those used inthe field of motor-vehicle lacquering, in particular of motor-vehiclerepair lacquering. The clear-lacquer coating compounds and one-coat toplacquer coating compounds contain water-thinnable binders. Thewater-thinnable binders are the conventional binders known to the personskilled in the art for this application purpose. They may, for example,be one-component or two-component water-thinnable binder systems.Preferred, however, are two-component binder systems.

Examples of one-component binder systems are those based onwater-thinnable polyurethane, polyacrylate, polyester and/or alkydresins. The one-component binder systems may, for example, be physicallyor oxidatively drying.

Examples of water-thinnable two-component, crosslinkable binder systemsare those based on hydroxyl-functional binders, such as, for example,polyurethane polyols, polyesterurethane polyols and/or polyacrylatepolyols and polyisocyanates, based on acetoacetyl-functional and(meth)acryloyl-functional binders, and based on(meth)acryloyl-functional binders or (meth)acryloyl-functional andglycidyl-functional binders and polyamines. Examples of the abovementioned binder systems are described more comprehensively inWO-A-94/03511, EP-A-358 979, EP-A-496 205 and DE-A-40 27 259.

It is likewise also possible to use water-thinnable binders that can becured, at least partially, by means of high-energy radiation, preferablyUV radiation. Preferably, these are free-radical-curable binders. Thepreferred free-radical-curing binders may be prepolymers, such aspolymers or oligomers that contain free-radical-polymerizable olefinicdouble bonds, in particular in the form of (meth)acryloyl groups in themolecule. The prepolymers may be combined with reactive thinners, i.e.reactive liquid monomers.

Examples of prepolymers or oligomers are (meth)acryloyl-functional(meth)acrylate copolymers, epoxy resin (meth)acrylates, polyester(meth)acrylates, polyether (meth)acrylates, polyurethane(meth)acrylates, unsaturated polyesters, unsaturated polyurethanes orsilicone (meth)acrylates with number-average molecular masses (Mn)preferably in the range from 200 to 10000, particularly preferably from500 to 3000, and having, on average, 2 to 20, preferably 3 to 10free-radical-polymerizable olefinic double bonds per molecule.

If reactive thinners are employed, they are used in quantities of 1 to50 wt. %, preferably of 5 to 30 wt. %, relative to the total weight ofprepolymers and reactive thinners. These are low-molecular-weight,defined compounds that may be monounsaturated, diunsaturated orpolyunsaturated. Examples of such reactive thinners are: (meth)acrylicacid and their esters, maleic acid and its half-esters, vinyl acetate,vinyl ether, substituted vinyl ureas, ethylene glycol and propyleneglycol di(meth)acrylate, 1,3- and 1,4-butanediol di(meth)acrylate, vinyl(meth)acrylate, allyl (meth)acrylate, glycerol tri-, di- andmono(meth)acrylate, trimethylolpropane tri-, di- and mono(meth)acrylate,styrene, vinyltoluene, divinylbenzene, pentaerythritol tri- andtetra(meth)acrylate, di- and tripropylene glycol di(meth)acrylate,hexanediol di(meth)acrylate, and also mixtures thereof.

Usable UV-curable binders are described, for example, in DE-A-41 33 290.

Lacquer undercoat coating compounds usable in the method according tothe invention are, for example, conventional lacquer undercoats known tothe person skilled in the art, such as those used in the field ofmotor-vehicle lacquering, in particular motor-vehicle repair lacquering.The lacquer undercoats may be water-based or solvent-based. Examples ofsolvent-based lacquer undercoats are those based on polyacrylate resinsand/or polyester resins, optionally in combination with melamine resinsand cellulose esters. Examples of water-based lacquers are those basedon physically drying polyurethane, polyurethane/urea, polyester,polyester urethane and/or polyacrylate resins and their modifications,such as, for example, acrylated or silicon-modified polyurethane resinsand/or polyester resins. Furthermore, water-based lacquers composed ofchemically crosslinking binder components, for example composed ofhydroxyl-group-containing binders and polyisocyanate curing agents, aresuitable.

Water-thinnable binders that can be at least partially cured by means ofhigh-energy radiation, preferably UV radiation, can likewise be used.Preferably these are free-radical-curable binders, such as those alreadymentioned above.

The binder systems mentioned here and suitable for lacquer undercoats,clear lacquers and one-coat top lacquers are only an exemplaryenumeration. The binder systems can likewise also be substantiallymodified and various crosslinking mechanisms may also be combined withone another, for example a curing by means of UV radiation can becombined with a further crosslinking mechanism. Examples of thelast-named combination are described in the still unpublished GermanPatent Application of the same Applicant P 198 187 35 and inWO-A-9800452 and DE-A-197 09 560.

The lacquer undercoat coating compounds and one-coat top lacquer coatingcompounds usable in the method according to the invention containcolouring and/or special-effect-producing pigments. Suitable as colourpigments are all conventional lacquer pigments of an organic orinorganic nature. Examples of inorganic or organic colouring pigmentsare titanium dioxide, micronized titanium dioxide, iron oxide pigments,carbon black, azo pigments, phthalocyanine pigments, quinacridone orpyrrolopyrrol pigments. Examples of special-effect-producing pigmentsare metal pigments, for example composed of aluminium, copper and othermetals; interference pigments, such as, for example, metal-oxide-coatedmetal pigments, for example titanium-dioxide-coated ormixed-oxide-coated aluminium, coated mica, such as, for example,titanium-dioxide-coated mica and graphite special-effect pigments. Inparticular, transparent pigments may also be present in the usable clearlacquers.

The coating compounds usable in the method according to the inventionmay furthermore contain water as well as organic solvents andconventional lacquer additives.

The organic solvents optionally present in the coating compounds areconventional lacquer solvents. These may originate from the productionof binders or be separately added. In the case of water-based coatingcompounds, they are preferably water-miscible solvents. Examples ofsuitable solvents are monohydric or polyhydric alcohols, for examplepropanol, butanol, hexanol; glycol ethers or glycol esters, for examplediethylene glycol dialkyl ethers, dipropylene glycol dialkyl ethers,each containing C1- to to C6-alkyl, ethoxypropanol, butyl glycol;glycols, for example ethylene glycol, propylene glycol and theiroligomers, N-methylpyrrolidone and ketones, for example methyl ethylketone, acetone, cyclohexanone; aromatic or aliphatic hydrocarbons, forexample toluene, xylene or linear or branched aliphatic C6-C12-hydrocarbons.

The coating compounds may furthermore contain conventional lacqueradditives. Examples of conventional lacquer additives are flow controlagents, rheology-influencing agents, such as highly dispersed silica orpolymeric urea compounds, thickeners, such as incipiently crosslinkedpolycarboxylic acid or polyurethanes, anti-foaming agents, wettingagents, anti-scratch agents, light stabilizers and curing accelerators.The additives are used in conventional quantities known to the personskilled in the art.

If binders curable by means of UV radiation are used, the top lacquercoating compounds additionally contain photoinitiators, for example inquantities from 0.1 to 5 wt. %, preferably from 0.5 to 3 wt. %, relativeto the total of free-radical-polymerizable prepolymers, reactivethinners and photoinitiators. Examples of photoinitiators are benzoinand benzoin derivatives, acetophenone and acetophenone derivatives, forexample 2,2-diacetoxyacetophenone, benzophenone and benzophenonederivatives, thioxanthone and thioxanthone derivatives, anthraquinone,1-benzoylcyclohexanol, organophosphorus compounds, such as, for example,acylphosphine oxides. The photoinitiators may be used alone or incombination.

In the case of two-component coating compounds, the interactive bindercomponents must be stored separately and can only be mixed with oneanother shortly before application.

In general, adjustment to spraying viscosity may be carried out, ifnecessary, prior to application with water or organic solvents.

The coating compounds in the method according to the invention may beapplied by conventional methods, preferably by means of sprayapplication.

Conventionally, the top coatings are applied from a colouring and/orspecial-effect-producing solvent-based or water-based undercoat coatingcompound and a water-based clear-lacquer coating compound or from awater-based pigmented one-coat top lacquer coating compound tosubstrates optionally coated with fillers and/or further coatingcompounds. Suitable as substrates are metal substrates and plasticsubstrates, in particular the substrates known in the automobileindustry, such as, for example, iron, zinc, aluminium, magnesium,stainless steel or their alloys, and polyurethanes, polycarbonates orpolyolefins. A filler layer is conventionally first applied to theoptionally pretreated and/or precoated substrates and cured.

The top lacquer coat can then be applied to the filler layer. In thecase of a top lacquer layer composed of a lacquer undercoat coatingcompound and a clear-lacquer coating compound, the lacquer undercoat isapplied first. The lacquer undercoat can be cured at room temperature orbe force-cured at, for example 40 to 80° C. The lacquer undercoat may,however, also be overlacquered in a wet-on-wet process with the clearlacquer, optionally after a drying phase and then cured together withthe clear lacquer. In particular if water-based lacquers are used, theprocedure may also preferably be to dry the applied lacquer undercoat bymeans of NIR radiation. The more precise procedure for NIR irradiationis explained below.

The clear lacquer is then applied. In accordance with the preferredembodiment, the application of the clear lacquer is followed by a dryingphase, for example within 5 to 45 minutes, preferably 15 to 40 minutes,at, for example, 10 to 80° C., preferably at room temperature. Thedrying phase is followed by the irradiation with NIR radiation. In thisprocess, the irradiation may be performed, for example, in a conveyorsystem equipped with an NIR source or with an NIR source that ispositioned in front of the object to be irradiated or the area to beirradiated.

The first-mentioned possibility is suitable in the case of repairlacquering of individual parts, in which connection the belt speed andconsequently the irradiation time can be varied. For example, beltspeeds of 1.0 to 7.0 m/min can be set, which may correspond, forexample, to irradiation times of from 2 to 20 s. The distance betweenNIR source and object surface may be, for example, 10 to 60 cm.

In the case of the second possibility, the NIR source is positioned infront of the object to be irradiated or the area to be irradiated. Theirradiation time may be, for example, 1 to 300 s and the objectdistance, for example, 5 to 60 cm.

Various object temperatures may be set by controlled selection of thevarious parameters, such as conveyor speed, irradiation time and objectdistance, and, of course, as a function of the radiation power of theNIR source used. For example, object temperatures from 80 to 150° C. canbe set.

After irradiation of the clear-lacquer coat with NIR radiation, curingis complete.

Only if binders curable by means of high-energy radiation are used is aUV irradiation also added. For this purpose, UV radiation sources havingemissions in the wavelength range from 180 to 420 nm, in particular from200 to 400 nm, are preferably used. Suitable UV radiation sources andthe UV technology are part of the prior art and are known to the personskilled in the art.

In the case of a top lacquer coating composed of a one-coat top lacquercoating compound, the one-coat top lacquer is applied to the substrate,preferably to the filler coat. The application of the one-coat toplacquer may preferably be followed by a drying phase, as describedabove. The drying phase is followed by radiation with NIR radiation. Inthis process, the irradiation can be performed, for example, in aconveyor system equipped with an NIR source or with an NIR source thatis positioned in front of the object to be irradiated or the area to beirradiated. If binders curable by means of high-energy radiation areused, a UV irradiation must also be added, as already described above.

An advantageous development of the method according to the invention isto cure the applied filler coat likewise with NIR radiation. In thiscase, it is possible, after applying, for example, an aqueous fillercoating compound, first to subject the filler coat, after an optionallyprovided drying phase, to a drying by irradiation with NIR radiation.The final curing can then be performed by a suitable curing method. Thefinal curing can be performed, for example, at room temperature orforced at higher temperatures, by irradiation with UV or IR or NIRradiation. Preferably the final curing is performed with UV or NIRirradiation.

It is, however, also possible to perform the curing of the filler coatafter an optionally provided drying phase only with a single NIRirradiation phase.

The method according to the invention is preferably used inmotor-vehicle lacquering and motor-vehicle parts lacquering, inparticular in motor-vehicle repair lacquering. It is, however, alsopossible, to use the method according to the invention in motor-vehicleseries lacquering, in particular in repair lacquerings in themotor-vehicle series lacquering.

Top lacquer coats having a uniform, reproducible quality with regard tointerlayer adhesion and surface properties can be achieved with themethod according to the invention even under varying externalconditions, in particular in the case of widely varying atmospherichumidity. Likewise, a uniform quality of lacquering is also ensured atcritical areas such as corrugations or edges. The top lacquer coatingshave a very good hardness and scratch resistance. After a short time,they can be polished very well. Also advantageous, of course, is theextremely short drying or curing time, respectively, that is achievedwith the method according to the invention. The extremely short dryingor curing time, respectively, is not substantially impaired by thedrying phase that is to be optionally provided and that occurs also inthe case of a conventional lacquering process with curing, for example,in an oven. The throughput times, for example, in a repair lacqueringworkshop can thereby be substantially shortened, which improves theefficiency of the workshop as a whole.

The time factor may also play a substantial role in repair lacqueringsin motor vehicle series lacquering.

The invention will be explained in greater detail by reference to thefollowing examples.

EXAMPLE 1

A water-based lacquer (produced according to DE-A-196 43 802, ProductionExample 4) was applied to a mudguard, coated with a commercialtwo-component (2C) filler coating compound, of a motor vehicle in aresultant dry film coat thickness of 13 to 15 μm. After a drying phaseof 25 minutes at room temperature, a water-based clear lacquer based onan OH-functional polyurethane resin and a polyisocyanate curing agent(produced according to WO-A-94/03511, Example 11) was applied in aresultant dry-film coat thickness of 50 μm. A drying phase of 35 minutesat room temperature was followed by irradiation with an NIR source (500W/cm²). The source/object distance was 10 cm and the irradiation timewas 6 s.

EXAMPLE 2

A water-based lacquer (produced according to DE-A-196 43 802, ProductionExample 4) was applied to a mudguard, coated with a commercial 2C fillercoating compound, of a motor vehicle in a resultant dry film coatthickness of 13 to 15 μm. A drying time of 10 minutes was followed byirradiation with a NIR source (500 W/cm²). The source/object distancewas 10 cm and the irradiation time was 6 s. Then a water-based clearlacquer based on an OH-functional polyurethane resin and apolyisocyanate curing agent (produced according to WO-A-94/03511,Example 11) was applied in a resultant dry film coat thickness of 50 μm.A drying phase of 35 minutes at room temperature was followed byirradiation with an NIR source (500 W/cm²). The source/object distancewas 10 cm and the irradiation time was 6 s.

Comparison Example

The procedure was analogous to Example 1, with the sole difference thatthe applied clear lacquer, after a drying phase of 35 minutes, was curedfor 60 minutes at 60° C.

Comparison of the Lacquering Results

The mudguards lacquered in accordance with Examples 1 and 2 exhibit ahomogeneous, uniform optical surface quality with regard to body, glossand flow, whereas these properties are formed unsatisfactorily atcorrugations and edges in accordance with the comparison example.

Example 1 Example 2 Comp. Example Polishing capability satisfactorysatisfactory Unsatisfactory after 30 minutes after 24 h Satisfactorysatisfactory satisfactory Hardness (finger- very good very good goodnail test) (immediately after curing)

What is claimed is:
 1. A method of lacquering a substrate whichcomprises the steps of: (a) spraying a lacquer coating compound on aprecoated substrate, wherein the lacquer coating compound is selectedfrom the group consisting of a clear lacquer coating compound and aone-coat top lacquer coating compound, wherein the precoated substrateis a motor-vehicle body or part thereof; and (b) curing the lacqueringcoating compound with NIR radiation having a wavelength range from 760to 1500 nm.
 2. The method according to claim 1, wherein said methodfurther includes subjecting the lacquer coating applied in step (a) to adrying phase, wherein said drying phase occurs before the lacquercoating is cured in step (b).
 3. The method according to claim 2,wherein the drying phase occurs for 5 to 45 minutes at a temperatureranging from 10 to 80° C.
 4. The method according to claim 2, whereinthe drying phase occurs at room temperature.
 5. The method according toclaim 1, wherein said method further includes subjecting the one-coattop lacquer coating compound to a drying phase, wherein said dryingphase occurs before the one-coat top lacquer coating is cured in step(b).
 6. The method according to claim 5, wherein the drying phase occursfor 5 to 45 minutes at a temperature ranging from 10 to 80° C.
 7. Themethod according to claim 5, wherein the drying phase occurs at roomtemperature.
 8. The method according to claim 1, wherein the lacquercoating compound comprises a water-based coating compound.
 9. The methodaccording to claim 1, wherein the clear lacquer coating compound of step(a) is applied to a lacquer undercoating, wherein said lacquerundercoating is cured before applying the clear lacquer coating compoundof step (a).
 10. The method according to claim 9, wherein the clearlacquer coating compound of step (a) is applied to the lacquerundercoating in a wet-on-wet process.
 11. The method according to claim1, wherein said method comprises repair lacquering of motor-vehiclebodies or parts thereof.
 12. The method according to claim 1, whereinthe lacquer coating compound is comprised of binders, said binders beingcurable by high-energy UV radiation.
 13. The method according to claim1, wherein said method further includes curing the lacquer coatingcompound of step (a) with UV radiation after being cured in step (b)with NIR radiation, said lacquer coating compound being comprised ofbinders curable by high-energy UV radiation.